Most ROMs were masked (built in the factory, ubchangeable), however there were at least two kinds of ROMs that were programmable. One used a fusable link for each bit, and would therefore not be reusable. This was primarily intended for personal use, and I believe it was built by Zengrange. There were also limited production custom ROMs that were built in the factory using ultraviolet EPROMs. If you are lucky, the UV window is exposed, if not you would have to crack open the ROM module case. I do not recommend opening up ROMs to check how they were made ... there are people around who are familiar with wich ROMs used UV EPROMs.